Hydraulic damper and elastic-return strut comprising such a damper

ABSTRACT

The hydraulic damper according to the invention has at least one restricted communication passage (47) between its two working chambers (8, 9) whose cross section is controlled by a viscous restriction valve (51) positioned via a control pressure of a fluid of a control damper with at least one secondary chamber (41, 42) delimited in a secondary cylinder (39) by a control piston (40), which cylinder and control piston are integrally attached, one to the main body (1), the other to the damping piston (4) of the damper, such that the damping law of the control fluid positioning said valve (51) is linked to the relative displacements of the main body (1) and damping piston (4). 
     Application to the equipping of elastic-return struts with integral damping, in particular for helicopter rotors and blades. FIG. 2.

This is a division of application Ser. No. 668,655 filed Mar. 13, 1991,U.S. Pat. No. 5,178,241.

FIELD OF THE INVENTION

The present invention relates to a hydraulic damper, in particular adamper of the type with linear damping by viscous restriction of a fluidwhich is preferably highly viscous, as well as to an elastic-returnstrut comprising such a damper, and more precisely of theelastohydraulic type with integral damping.

More particularly, the damper proposed by the invention has a structurewhich enables it to adjust its own damping in accordance with anoptimised law which is a function of only the damped relativedisplacement. The strut proposed by the invention is particularlyintended for equipping the rotors of aerodynes with rotary wing unitsand, more especially helicopter rotors.

PRIOR ART

Servocontrolled dampers have already been known for a long time and usedparticularly in the field of aeronautics, as is disclosed, for example,by the document FR-950,592, regarded as state of the art and relating toa self-adjusting damper for the front wheel of aeroplane three-wheellanding gear.

The device proposed in this document essentially comprises a maincylinder, in which a piston is mounted movably in translation and in aleaktight manner in said cylinder. The stroke of this piston is dampedby the compression of a liquid, such as oil, which the movement of saidpiston causes to pass from one chamber to another of the main cylindervia a diaphragm. This diaphragm is associated with an adjustment fingerwhich, according to its positioning, more or less closes the orifice ofsaid diaphragm and thus adjusts the opening cross section of the latter.The positioning of this adjustment finger is controlled by theantagonistic actions of a spring which pushes said adjustment fingertowards a position of maximum opening of the orifice of said diaphragm,and of the pressure of a fluid derived from the circuit supplying thebrakes of the wheel.

By virtue of such a structure, the adjustment of the damper is directlysubject to the braking pressure developed, for example on landing.However, it should be noted that, if such a device effectively permitsautomatic adjustment of damping, this adjustment is not directlycontrolled by the damping stroke, but by an element or parameterexternal to the damper.

A damper is also known, from Patent FR-1,003,247, which has twocylindrical chambers each filled at least partially with a liquid, suchas oil, passing from one to the other of the two chambers through theaction of the movement of a damping piston, via a damping orifice. Thisdamping orifice is associated with a valve which is intended to closeit, said valve being actuated by the pressure of an external fluid.

Such a device also makes it possible to achieve servocontrol of thedamper. The damping law is, however, in this case also, a function of aparameter external to the damper and is not directly a function of thestroke of the damping piston in its main cylinder.

Patent FR-1,165,327 discloses an adjustment system for suspensiondampers which essentially comprises, in a main body, a slidably mountedmain piston. Said piston has two communication passages passing throughit between the two chambers it delimits in the main body. These passagesare themselves associated with valves closing them, respectively, duringphases of expansion or of delivery. This main piston is mounted on anactuating rod associated with at least two complementary elements whichtogether delimit a secondary chamber with a variable volume. One ofthese elements is fixed relative to said rod. The other can sliderelative to the latter and is intended to compress a spring mountedabout said rod, said spring acting on at least one of said valves andpushing it against its seat. A channel emerges into this secondarychamber and extends in the rod, feeding said secondary chamber with acontrol fluid whose pressure adjusts the load of the valve or valves,and thus the damping.

However, the adjustment thus obtained is, in this case also, an externaladjustment, remotely controlled, which is in no way influenced by therespective positions of the damping piston and of the main body.Moreover, it should be noted that, in such a device, the control fluiddoes not act directly on the valves with which it is associated, butacts on them via multiple parts which increase the complexity of theassembly and of the functioning of the device.

As an example of an externally servocontrolled damper, the vehicle oildamper disclosed by document FR-1,194,294 should also be mentioned, thefunctioning of which is substantially similar to that of the dampersdescribed above, the cross section of the passages for restricting theoil being controlled by adjustment valves whose positioning iscontrolled by an external member.

The subject of Patent FR-1,323,746 is a damper whose damping is directlyservocontrolled by the intensity of the oscillations to which saiddamper is subjected. This damper conventionally comprises a main dampingpiston which is movable in a main damper cylinder, the width of saidpiston having main passages passing through it which permit the flow ofthe fluid between the two working chambers delimited by said piston inthe main cylinder. The rod on which the piston is mounted comprises,inside, a bore which acts as a housing for a slide valve. The body ofsaid valve is pierced axially by a central orifice, the two auxiliarychambers delimited in said bore by said valve each being respectivelyplaced in communication via auxiliary passages passing through the wallsof said bore, with one of the two working chambers on each side of themain piston. The position of said valve in its bore controls the openingcross sections of said passages and thus the flow of the damping fluidfrom one working chamber to the other, via the auxiliary chambers.

Such a structure enables the damper to adapt itself, on its own, to thestresses it experiences. However, the damping law achieved depends onthe relative pressures in the two working chambers and not directly onthe respective positions of the damping piston and of the main cylinder.Damping may in particular be of differing intensities for one and thesame relative position of the damping piston and of the main cylinder.Moreover, the damping law is influenced by the cross section of theauxiliary passages which varies according to a decreasing friction ofthe loss of charge at the slide valve without this loss of chargecontrolling the variation in the flow cross section via the mainpassages of the damper.

The use of a slide valve to lower the operating pressures of a damper isfurther described in Application FR-2,597,952, which discloses the useof such a slide valve in order to open an auxiliary passage of verylarge cross section when the passage cross section of the mainrestriction ports of the damper is insufficient to prevent criticalvariations in pressure in the damper.

As for the struts, elastic-return struts with integral damping, ofhydroelastic type, have already been described, particularly in theApplicant's Patents FR-2,592,696 and FR-2,629,163. Such struts areparticularly intended, on the lifting rotors of a helicopter with anarticulated-type boss, to damp the angular oscillations of the blades inthe plane of rotation of the rotor (drag damping), by virtue of ahydraulic damping device. At the same time, these struts provide anenergetic elastic return of the blades in drag, by the deformation of amass of flexible material.

Such struts conventionally comprise essentially two rigid members eachequipped with articulation means intended to connect one of the rigidmembers to a first part, such as a blade or a member for joining saidblade to a rotor boss, and the other rigid member to a second part, suchas the rotor boss. These struts also comprise at least oneelastic-return member comprising a mass of deformable material attachedto the two rigid members and intended to be deformed when said rigidmembers are displaced relative to one another, and to exert anelastic-return action on said rigid members which tends to bring themback into an initial relative position. Moreover, they are equipped withat least one hydraulic damper, comprising two working chambers with avolume which is variable in the opposite direction, which contain arelatively viscous hydraulic fluid intended to pass from one to theother of said chambers via at least one restricted passage communicatingbetween said working chambers, when said rigid members are displacedrelative to one another, in order to produce a damping effect on therelative displacement of said rigid members.

SUMMARY OF THE INVENTION

An object of the invention is to propose a damper which makes itpossible to achieve an optimised damping law which is directly afunction of the damped relative displacements. To this end, theinvention proposes, in particular, a device which permits control of theopening cross section of at least one restricted passage of the damper,via the hydraulic fluid of a second damper substantially incorporated inthe main damper, the relative displacement of the elements of thissecond damper being controlled directly by the damping stroke of themain damper.

Such a damper is more particularly, but not exclusively, adapted forstruts of the abovementioned type, in respect of which it is intended toachieve an optimised damping law, with damping gradients which differaccording to the value of the dynamic displacements of the two memberswith which each strut is fitted.

A first subject of the invention is thus a hydraulic damper comprising,in a main body, two working chambers with a volume which is variable inthe opposite directions, said working chambers containing a main viscousfluid intended to pass from one to the other via at least one restrictedcommunication passage, through the action of the thrust of a dampingpiston being displaced in said body, the opening cross section of atleast one communication passage being controlled by the positioning,relative to said passage, of a viscous restriction valve on which acontrol fluid acts, this hydraulic damper being defined in that thecontrol fluid is contained principally in at least one secondary chamberdelimited in a secondary cylinder by a control piston mounted slidablyin said cylinder, said secondary chamber being associated with a channelfor conveying the control fluid from said secondary chamber as far asthe viscous restriction valve through the action of the control piston,said secondary cylinder and said control piston being integrallyattached, one to the main body, the other to the damping piston. Theinvention also proposes a hydraulic damper in which the opening of therestricted passage or passages is controlled by controlling the strokeof the viscous restriction valves with the aid of a pressurised controlfluid delivered by a hydraulic generator incorporated in the damper andsensitive to the relative displacements of the two fastenings of thedamper. When the damper is linear, the hydraulic generator is linked tothe relative linear displacements of the two fastenings between whichthe damper is mounted, and this hydraulic generator comprises thecontrol piston linked to a fastening of the damper, and mounted slidablyadvantageously with a viscous restriction choke in the secondarycylinder, linked to the other fastening of the damper, in order todeliver the control fluid at an average pressure which is sufficient tocontrol the displacements of the viscous restriction valve or valves.

The control piston is advantageously integrally attached to the mainbody, the secondary cylinder being integrally attached to the dampingpiston. The control fluid may be contained mainly in two secondarychambers delimited, in the secondary cylinder, by the control pistonmounted slidably in said cylinder. It is thus possible totally tointegrate the device for controlling the viscous restriction valves inthe general construction of the damper.

In an advantageous embodiment, the damper comprises, in its main body, adamping cylinder in which the two working chambers are delimited by thedamping piston, said damping piston being mounted on a main movable rodpassing through at least one of the working chambers, the secondarycylinder extending at least partially longitudinally into the mainmovable rod, the control piston being mounted on a secondary rod whichis movable in longitudinal translation inside said secondary cylinderand integrally attached to the main body. It can also comprise a deviceto compensate for the thermal expansions of the hydraulic fluid and/orthe variations in the volumes containing said fluid, comprising at leastone compensation chamber associated with a compensation circuit housedat least partially in the main movable rod, the secondary movable rodsliding at least partially inside the main movable rod, in thecompensation circuit. The compensation circuit may extend partially intothe secondary movable rod and emerge from said secondary movable rod inthe portion of said compensation circuit which is located inside themain movable rod.

Still in an advantageous manner, at least one secondary chambercommunicates via a supplementary channel with the compensation circuit,which permits the absorption in the compensation chamber of the thermalexpansions of the control fluid which is then the same fluid as theviscous fluid of the damper. The control piston may, in particular, bemounted in the secondary cylinder with a viscous restriction play, thesupplementary channel being disposed radially in said control piston andpermitting the communication of each of the two secondary chambers withthe portion of the compensation circuit which extends into the secondarymovable rod.

The secondary movable rod preferably extends into a first workingchamber from the main body, into the portion of the compensation circuitwhich is located in the main movable rod, passing through the secondarycylinder, the main movable rod extending beyond the damping piston intothe compensation chamber, passing through the second working chamber,the portion of the compensation circuit housed in said main movable rodemerging into said compensation chamber.

The damper may, in particular, comprise an auxiliary chamber connectedto the compensation chamber via at least one compensation channel, theworking chambers being disposed between said compensation chamber andthe auxiliary chamber, said auxiliary chamber having the secondarymovable rod passing through it. This auxiliary chamber may have the mainmovable rod passing at least partially through it.

In an advantageous embodiment, one of the ends of the main movable rodis displaced in said auxiliary chamber, the compensation circuitextending into the secondary movable rod and into the main movable rodin order to emerge, towards one of its ends, from the secondary movablerod into the auxiliary chamber and, towards its other end, from the mainmovable rod into the compensation chamber. The compensation chamber may,in particular, be at least partially delimited on the inside by aflexible wall in the form of bellows.

In a further equally advantageous embodiment, the main movable rodpasses right through the auxiliary chamber, a compensation channelextending into the main body. The auxiliary chamber may be includedbetween a working chamber and a charging chamber in which one of theends of the main movable rod is displaced, at least one wall separatingsaid auxiliary chamber and said charging chamber, said wall having arestricted communication orifice passing through it. The chargingchamber may have the secondary movable rod passing through it.

Still preferentially, each of the two secondary chambers is associated,via a transportation channel, with a viscous restriction valve. Thedamper may, in particular, comprise, between the two working chambers,two restricted communication passages and two viscous restriction valveseach controlling, respectively, via its positioning, the opening crosssection of each of said communication passages, each of these two valvesbeing associated, respectively, with one of the secondary chambers.

A viscous restriction valve is advantageously in balance between, on theone hand, the positioning pressure exerted on it by the control fluidand, on the other hand, a return spring, in particular a compressionspring. A viscous restriction valve may, in particular, be a shuttermounted slidably in a bore, in which it slides in a leaktight manner,said bore having a communication passage between the two workingchambers passing through it at the level of said valve, said valvecomprising a groove intended to connect together, inside the bore, thetwo ends of said passage and to adjust, according to the position ofsaid groove relative to said ends, the opening cross section of saidpassage, one of the two portions of the bore which are disposed oneither side of said valve being directly connected via a transportationchannel, to a secondary chamber. In the rest position, a viscousrestriction valve may, for example, substantially close the entirecommunication passage with which it is associated.

The damper may also comprise a supplementary or permanent restrictedcommunication passage, with a constant cross section for example,intended to permit the passage of the main fluid from one to the otherof the two working chambers when the other communication passage(s)is(are) substantially closed by the viscous restriction valve(s) withwhich it (they) is (or are) associated.

Still preferentially, each secondary chamber can be associated with atransfer circuit permitting a flow of fluid from one of the workingchambers towards said secondary chamber, when said secondary chamberfills up. Each working chamber may be connected, via a transfer circuit,to a first secondary chamber and connected, via a complementary circuit,to the second secondary chamber, so as to permit the transfer, when saidsecond secondary chamber empties, of substantially the quantity of fluidby which said second secondary chamber is emptied, from said secondsecondary chamber towards said first secondary chamber, by means of amain chamber. More particularly, that one of the two portions of boredisposed on either side of the viscous restriction valve which slides inthis bore and which is opposite the transportation channel connectingsaid bore to a secondary chamber, emerges, via complementary conduits,into the working chamber connected via a transfer circuit to the otherof said second chambers, thus producing, with said transportationchannel, a complementary circuit.

In an advantageous manner, a working chamber associated with a secondarychamber via a transfer circuit fills at the same time as said secondarychamber.

Still in an advantageous manner, at least a portion of the transfercircuit is associated with a differential valve intended to close itwhen the secondary chamber associated with said transfer circuitempties. A differential valve may, in particular, be closed through theaction of the pressure of the fluid in the working chamber in which thetransfer circuit associated with said differential valve emerges whensaid working chamber empties. The differential valves of the transfercircuits of each of the two secondary chambers are functionally and/orstructurally combined with one another, one being in the closedposition, bearing on its seat, while the other is in the open position.

A transfer circuit preferably emerges into the secondary chamber withwhich it is associated, via the transportation channel associated withsaid secondary chamber. The body or main cylinder, the main rod, thesecondary cylinder and the secondary rod can have coincident axes. Thebore of a viscous restriction valve can extend into the damping piston,perpendicularly to the axis of the main rod. The differential valves canalso be disposed in the damping piston, symmetrically relative to theviscous restriction valves and perpendicularly to the axis of the mainrod.

A further subject of the invention is an elastic-return strut withintegral damping, of elastohydraulic type, in particular for controllingthe alternated angular movements of the blades of a rotor of an aerodynewith a rotary wing unit in the plane of rotation of said blades,comprising:

two rigid members each equipped with articulation means intended toconnect one of the rigid members to a first part, such as a blade or amember for joining said blade to the boss of the rotor, and the otherrigid member to a second part, such as said boss of the rotor;

at least one elastic-return member comprising a sleeve in an elasticallydeformable material adhering in a leaktight manner via its inner andouter lateral surfaces, respectively, between inner and outer rigidtubular sections respectively, substantially coaxial with respect to theaxis of the strut and each integrally attached to one, respectively, ofthe two rigid members, so that the sleeve is deformed in shear, when thetwo rigid members are displaced relative to one another substantiallyaccording to the general axis of the strut and when the sleeve exerts anelastic return action on said rigid members which tends to bring themback into an initial relative position; and

a hydraulic damper according to the invention, whose main body forms oneof the two rigid members, the damping piston of said damper beingintegrally attached to the other of said rigid members.

The secondary movable rod and the main movable rod may be mounted, oneon the articulation means connecting one of said rigid members to thefirst part, the other on the articulation means connecting the other ofsaid rigid members to the second part.

In an advantageous embodiment, the strut comprises a sleeve adhering viaits inner surface to the outer surface of a tubular wall of the mainbody of the damper.

In a further embodiment, also advantageous, it comprises two elasticsleeves adhering via their inner surface to the outer tubular surface ofsections integrally attached to the main movable rod and, via theirouter surface, to the inner tubular surfaces of sections integrallyattached to the main body of the damper. The elastic sleeves may,respectively, at least partially delimit the compensation chamber andthe auxiliary chamber. The secondary movable rod may be connected to themain movable rod via at least one flexible elastic-return wall. Theflexible elastic-return wall may have a frustoconical form widening outfrom a sheath in which the secondary movable rod is mounted as far asthe inner wall of a tubular section.

Preferably, the damping cylinder, the main rod, the secondary rod andthe secondary cylinder have coincident axes.

BRIEF DESCRIPTION OF THE DRAWINGS

The description which follows is given with reference to the appendeddrawings. It is purely illustrative and has no limiting character.

In these drawings:

FIG. 1 is a diagrammatic representation illustrating the functioningprinciple of the inner control device of a damper according to theinvention;

FIG. 2 is a sectional view of a hydroelastic strut according to a firstembodiment, equipped with a damper according to the invention;

FIG. 3 is a sectional view according to the line III--III in FIG. 2;

FIG. 4 is a sectional view according to the line IV--IV in FIG. 2;

FIG. 5 is a sectional view, similar to FIG. 2, of a hydroelastic strutaccording to a second embodiment, equipped with a damper according tothe invention;

FIG. 6 is a sectional view according to the line VI--VI in FIG. 5;

FIG. 7 is a sectional view according to the line VII--VII in FIG. 4;

FIG. 8, finally, is a graph illustrating a damping law which can beobtained with a damper according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 to 4 and 7 show that a hydroelastic strut comprising a damperaccording to a first embodiment of the invention essentially comprisestwo rigid bodies 1 and 2 which are movable relative to one another, therigid body 1 being the main body of the damper, the rigid body 2 beingintegrally attached to a main rod 3, which supports a damping piston 4which is movable in translation in a damping cylinder 5 provided in themain body 1.

The main rod 3 passes right through the damping cylinder 5. Its endopposite its end via which it is fixed to the rigid body 2 is displacedin an auxiliary chamber 6 provided in the main body 1 towards one of itsends. This auxiliary chamber 6 has a substantially cylindrical form. Itsdiameter is slightly greater than the external diameter of the main rod3 at the level of said end. The piston 4 is a disk whose outer diametercorresponds substantially to the inner diameter of the main cylinder 5in which it slides in a substantially leaktight manner, the seal betweensaid piston 4 and the main cylinder 5 being provided by an annulardynamic seal 7 surrounding the periphery of said piston 4. In the maincylinder 5, this damping piston 4 delimits two working chambers 8 and 9,that of these two chambers which is nearest to the auxiliary chamber 6being denoted by 8, the other by 9.

A compensation chamber 10 is also disposed in the main body 1 in theextension of the auxiliary chamber 6 and of the working chambers 8 and9. This compensation chamber 10 has the main rod 3 passing through it,of which the part disposed opposite the working chamber 9 relative tosaid chamber 10 is its end via which it is integrally attached to therigid body 2. This compensation chamber 10 is partially delimited on theinside via a flexible bellows wall 11 which has a form in revolutionwhich is centered on the axis of the main rod 3 and widens substantiallyfrom the end of said chamber 10, which is the furthest away from theworking chamber 9, towards it other end. This compensation chamber 10and the bellows 11 are disposed in a cylindrical recess 12 delimited inthe main body 1 via a tubular wall 13 which also delimits the maincylinder 5. Said cylindrical recess 12 is open at its end opposite theworking chamber 9 and is delimited at its other end via a partition 14which separates said recess 12 from said main cylinder 5.

This partition 14 has a bore 15 passing coaxially through it, said borehaving the main rod 3 passing through it. A ring 16 forming a bearingand annular dynamic leaktight seal is disposed between the innercylindrical wall 17 of said bore 15 and the main rod 3. This ring 16,made from a friction material, such as bronze, for example, is held inplace axially relative to the bore 15 by being gripped between anannular rim 18 surrounding the wall 17 on the inside, at its end on theside of the compensation chamber 10, and a stop part 19. The stop part19 is flat and annular and is disposed so as to bear on the surface ofthe partition 14, on the side of the working chamber 9. It is heldrelative to said partition 14 by means of a screw 20 passing throughsaid partition 14 and whose screw thread interacts with thecomplementary screw thread with which are equipped tapped bores passingthrough an annular housing part 21 intended to hold, via one of itsedges, the flexible wall of the bellows 11 relative to the partition 14,by gripping said edge between the partition 14 and the housing part 21.The head 22 of a screw 20 bears on the stop part 19 by means of aninserted thin plate 23 which is annular and surrounded by holding lugs,said head 22 being itself disposed in the working chamber 9.

The working chamber 8 is delimited at its end opposite the dampingpiston 4 by a base 24 with a bore 25 passing through it, in which it ispossible to slide the main rod 3. This bore 25 is surrounded by thecylindrical wall delimiting the auxiliary chamber 6. It is alsoassociated with a ring 26 forming a bearing and annular dynamicleaktight seal which, interacting with the wall of the auxiliary chamber6 and the main rod 3, ensures leaktightness between the working chamber8 and said auxiliary chamber 6. The dynamic seal 26 is held in place ina shoulder 27 made in the inner wall of the bore 25 by being grippedbetween an annular lateral wall of said shoulder 27 and a stop part 28held in place on the base 24, in the chamber 8, by means of theinteraction of a screw 30 whose screw threads interact with the screwthreads of tapped bores made in the base 24 and whose heads 31 bearagainst said stop part 28 by means of an inserted thin plate 29. Theheads 31 of the screws 30 are disposed inside the working chamber 9. Thebearing/seal 26 permits the sliding of the main rod 3 in the bore 25 andprovides leaktightness between the auxiliary chamber 6 and the workingchamber 9.

The auxiliary chamber 6 has a secondary rod 32 passing through it, whichrod extends from the base 33 of said auxiliary chamber 6 on the sideopposite the chamber 8, into the main rod 3. In its part which extends,from the damping piston 4, into the working chamber 8 and into theauxiliary chamber 6, the main rod 3 has a tubular recess 34 passingthrough it, the axis of which is coincident with the axis of said rod 3.This tubular recess 34 is extended beyond the damping piston 4, in theportion of the rod 3 which extends in the working chamber 9 and into thecompensation chamber 10, via a tubular channel 35 whose inner diameteris slightly larger than the outer diameter of the secondary rod 32. Thischannel 35 is central on the axis of the main rod 3, the secondary rod32 extending partially into said channel 35 in which its free end isdisposed and in which it can slide relative to the main rod 3.

A separation disk 36 is disposed in the recess 34, toward the end of therod 3 which is located in the auxiliary chamber 6, the outer diameter ofwhich disk corresponds substantially to the inner diameter of saidrecess 34. This disk 36 has the secondary rod 32 passing through it andis associated with annular leaktight seals 37 and 38 which,respectively, on the one hand, provide the static leaktightness betweensaid separation disk 36 and the inner wall of the recess 34, and, on theother hand, the dynamic leaktightness between said disk 36 and thesecondary rod 32. A secondary cylinder 39, whose outer diametercorresponds substantially to the inner diameter of said recess 34 andinside which the rod 32 slides, is disposed in the recess 34, betweenthe base of said recess, into which the channel 35 emerges, and the disk36. In its central part, this rod 32 carries a piston 40 which, in saidsecondary cylinder 39, delimits two secondary chambers 41 and 42, thechamber 41 being delimited by said secondary cylinder between saidpiston 40 and disk 36, the chamber 42 being the other of the twochambers.

The secondary piston 40 is mounted slidably in the secondary cylinder 39with a peripheral viscous restriction play. The secondary rod 32comprises, inside, a secondary channel 43 which extends in the length ofsaid rod 32, from the auxiliary chamber 6 as far as the free end of saidrod 32 disposed in the channel 35. The secondary piston 40 has asupplementary channel 44 passing radially through it, which channel, byvirtue of the viscous restriction play, places the inside of thesecondary cylinder 39 in communication with the secondary channel 43.The channels 35 and 43 are also associated with channels 45 and 46which, respectively, pass radially through the main rod 3 and thesecondary rod 32, in order to provide the communication between thecompensation chamber 10 and the auxiliary chamber 6 by means of thechannels 35 and 43. The auxiliary chamber 6, the working chambers 8 and9, the compensation chamber 10, the channels 35 and 43 to 46 as well asthe secondary chambers 41 and 42 are filled with a highly viscous oil.The auxiliary chamber 6, the compensation chamber 10 and the channels35, 43 to 46 form the elements of a compensation device intended tocompensate both for the thermal expansions of the hydraulic fluid andthe variations in the volumes of the chambers which contain said fluid.

The thickness of the damping piston 4 has two communication passages 47and 48 passing through it which are intended to serve as a restrictedcommunication passage between the two working chambers 8 and 9. Thesetwo passages 47 and 48 are substantially tubular, their axes extendingin the thickness of the piston 4, in a cluster in the same plane, thisplane being substantially tangent to the outer surface of the rod 3, thecommon axis of said piston 4 and of the rod 3 being contained in theintermediate plane of these two straight lines. The ends of thesecommunication passages 47 and 48 which emerge into the chamber 9 arecloser to one another than the ends of said communication passages whichemerge into the chamber 8 (see FIG. 7).

The piston 4 also has two cylindrical bores 49 and 50, whose axes areperpendicular to the axis of the rod 3 and to the plane of the axes ofthe communication passages 47 and 48, passing through its inside. Thesetwo bores 49 and 50 have the communication passages 47 and 48 passingthrough them, respectively. In the piston 4, they are closer to theworking chamber 9 than the chamber 8. Cylindrical viscous restrictionsvalves, denoted, respectively, by 51 and 52 are disposed inside thesebores 49 and 50, substantially at the level of the communicationpassages 47 and 48. These valves 51 and 52 are, respectively, eachassociated with a coiled spring 53 and 54 extending in the bore 49 or 50with which the valve on which they act is associated, being held inplace in said bore 49 or 50 by means of a capsule, denoted,respectively, by 55 and 56, in which said spring 53 or 54 is embedded atits end opposite the valve 51 or 52. Each capsule 55 or 56 is disposed,respectively, in a housing 57 or 58 extending the bore 49 or 50 withwhich it is associated, at its end disposed from the side opposite thecommunication passage 47 and 48 relative to the rod 3. The innersurfaces of these housings 57 and 58 are equipped with screw threadswhich interact with the screw threads with which the capsules 55 and 56are equipped on their outer surface, it being possible for the positionof a capsule 55 or 56 in its housing 57 or 58 thus to be adjusted by anoperator in order to adjust the opening pressure of the valve 51 or 52.The housings 57 and 58 emerge at their end opposite the bore outside thedamping piston 4. The capsules 55 and 56 are, moreover, respectivelyequipped, on the surface of their base directly opposite the wall 13,with a slot 55a or 56a intended to interact with an external tool inorder to permit said adjustment of the capsule in its housing, forexample to interact with a screwdriver. The walls of the piston 4 whichrespectively partially delimit the chambers 8 and 9 are connected to thecylindrical lateral wall by a frustoconical form, at the level of whichthe housings 57 and 58 emerge outside the piston 4. The housings 57 and58 are thus both in communication with the working chamber 9.

Each viscous restriction valve 51 or 52 is equipped with a groovedenoted, respectively, by 59 and 60, which surrounds it annularly in itscentral part. Said groove 59 or 60 has a cross section, in a planepassing through the axis of the bore, in the form of a half-circle whosediameter corresponds substantially to the diameter of the restrictedcommunication passages 47 and 48. Said groove 59 or 60 is intended toconnect together, inside the bore 49 or 50 with which its valve 51 or 52is associated, the two ends of the communication passage 47 or 48 whichpasses through said bore 49 or 50, when said valve 51 or 52 ispositioned in its bore 49 or 50 so that the groove 59 or 60 with whichit is equipped is located at the level of said communication passage 47or 48. At its end disposed on the side opposite the spring 53 or 54,relative to the valve 51 or 52 with which it is associated, thereemerges, in each bore 49 or 50, a transportation channel 61 or 62 whichextends from said end as far as the secondary chamber 41 or 42,respectively, a portion of the transportation channel 61 extending intothe length of the wall of the secondary cylinder 39 in order to emergeinto the secondary chamber 41, substantially at the level of theseparation disk 36 which delimits said chamber 41 at one of its ends.

As is clearer in FIGS. 1 and 3, a transfer channel 63 or 64,respectively, extending between said transportation channel 61 or 62and, respectively, the working chamber 9 or 8, emerges in the centralpart of each of these transportation channels 61 and 62. The opening ofeach of these transfer channels 63 or 64 is controlled by a differentialvalve 67 or 68 associated with a seat 65 or 66. The two valves 67 and 68are spherical valves and are connected together by a joining rod 69.Each of these valves 67 or 68 is movable relative to its seat 65 or 66,respectively, in a housing 70 or 71 in which the portion of the transferchannel 63 or 64 which communicates directly with the chamber 9 or 8emerges. The arrangement of these valves 67 and 68 is such that theworking surface of each one directly associated, respectively, with thechamber 9 or 8 is greater than the surface of each one associated,respectively, with the chamber 41 or 42. The housing 70 or 71 isdelimited partially by the inner walls of a cap 72 or 73 mounted byscrewing in a bore of the damping piston 4, directly opposite theorifice which passes through the seat 65 or 66 and where the part of thetransfer channel 63 or 64 which communicates directly with thetransportation channel 61 or 62 emerges. The differential valves 67 and68 as well as the seats 65 and 66 with which they are associated aredisposed symmetrically relative to the intermediate plane of thecommunication passages 47 and 48. The portion of a bore 49 or 50 inwhich the spring 53 or 54 is disposed is connected via a conduit 74 or75, respectively, to the working chamber 8 or 9. The transfer channel 63or 64 and the transportation channel 61 or 62 together form,respectively, a transfer circuit connecting the working chamber 9 or 8to the secondary chamber 41 or 42.

The thickness of the damping piston 4 also has a third communicationpassage 76 passing through it, which passage extends into said piston 4from one to the other of the two working chambers 8 and 9 parallel tothe axis of the rod 3 and substantially in the intermediate plane of thetwo communication passages 47 and 48. The inner diameter of thiscommunication passage 76 is much smaller than the inner diameter of thecommunication passages 47 and 48. This passage 76 thus forms a permanentrestriction passage of constant cross section.

Referring to FIG. 2, it can be seen that the damper describedhereinabove is incorporated into a strut structure which comprises, in aknown manner, a rubber sleeve 77 which deforms elastically, essentiallyin shear, and which is molded between the tubular outer surface of thewall 13 of the main body 1 and the inner wall of a tubular section 78integrally attached to the body 3. This section 78 is fixed by a flange79, which terminates it, against the flange 80 of a plate 81 of therigid body 2, with the aid of twelve screws 82 of which the screw threadinteracts with the screw threads with which the bores made in the flange79 are equipped, and the head of which bears on the outer surface of thecomplementary flange 80. The plate 81 is centred on the tubular section78, by means of an inner cylindrical collar 83.

At its end on the side opposite the working chambers 8 and 9 and thecompensation chamber 10, the plate 81 carries a fixing lug 83 equippedwith a pivot eye 84 and with a pivot 85 for articulating the strut, forexample on the boss of the rotor (not shown). The main rigid body 1 isequipped, in the extension of the working chambers 8 and 9 and of theauxiliary chamber 6, at its end opposite the plate 81, with a fixing lug86 in which is mounted a pivot 87 for articulating the strut, forexample on the root of a rotor blade (not shown) or on a sleeve or ashell (also not shown) for joining the blade to the boss.

The cylindrical wall 13 of the main rigid body 1 terminates, beyond thesleeve 77, at the level of its end opposite to its end via which itemerges in front of the plate 81, in a flange 88 interacting with acomplementary flange 89 in order to hold, relative to the portion of themain body 1 which carries the sleeve 77, the outer portion of said mainbody 1 in which the auxiliary chamber 6 is mounted and which carries thepivot 87 and the lug 86 which is associated therewith, an outer wall ofsaid portion defining the base 24 of the working chamber 8. These twoflanges 88 and 89 are integrally attached to one another by means of ascrew 90 whose screw thread interacts with the screw thread with whichbores passing through the flange 88 are equipped and whose head bears onthe surface of the flange 89 which is furthest away from the sleeve 77.The flange 89 is positioned relative to the flange 88 by means of acollar 91 with which it is equipped, said collar 91 being housed in anannular recess 92 made in the inner wall of the flange 88 in order toreceive it. Between the outer surface of said collar 91 and the innersurface of the wall of the flange 88 defining said recess 92 is disposeda static annular leaktight seal 93. The main body 1 is also equipped, atthe level of said flanges 88 and 89, with a filling and purging screw 94which closes or opens a vent emerging into the main cylinder 5,substantially in the working cylinder 8.

The secondary rod 32 is integrally attached to the base 33 of theauxiliary chamber 6 by conventional securing means such as a transversepin 32a. Said secondary rod 32, the main rod 3, the main cylinder 5, thesecondary cylinder 39 and the auxiliary chamber 6 have coincident axes.The rod 3 is integrally attached to the central boss carrying the plate81, by interaction with an outer screw thread with which it is equippedat its end at the level of said plate 81, with, on the one hand, theinner screw thread of a bore 95 in which said rod 3 is embedded, andwith, on the other hand, the screw thread of an outer counternut 96.This counternut 96 can be gripped with a suitable tool engaged betweenthe plate 81 and the tubular section 78 by placing the main rod 3 upagainst the partition 14.

The end of the bellows wall 11 delimiting the compensation chamber 10,opposite the end of said wall which is mounted on the partition 14, isintegrally attached to a ring 97 in which the main rod 3 can slide, saidring 97 being associated with a dynamic seal 98 ensuring theleaktightness of said compensation chamber 10. A reference rod 99, whichextends from said ring 97 as far as the plate 81 which it passesthrough, is mounted on said ring 97, said reference rod 99 beingintended to permit the control of the filling of the compensationchamber 10.

The strut described above is used in the manner presented hereinbelow.When the rigid bodies 1 and 2 have, through the action of the rotationof the blades and of the rotor, a movement relative to one another andmove away from their balance position, they are returned towards thisposition by means of the elastic sleeve 77, the movement also beingdamped by the assembly forming the damper.

By way of example, as has been shown by the arrows in FIG. 1, thedamping piston 4 is displaced in the main cylinder 5 so as to reduce thevolume of the chamber 9, which is under compression, and to increasethat of the working chamber 8, which is at reduced pressure, whichcorresponds to a tensile stressing of the strut and of its damper, theoil contained in the secondary chamber 41 is pushed back by thesecondary piston 40 towards the valve 51 by means of the transportationchannel 61, subject to a leakage flow passing from the secondary chamber41 to the other secondary chamber 42 via the permanent viscousrestriction play around the piston 44.

As long as the relative displacement of the bodies 1 and 2 remainssufficiently small, the viscous restriction valve 51 is held under thepressure of the spring 53 in the base of the bore 49. The communicationpassage 47 then remains closed by said valve 51. At the same time, sincethe secondary chamber 42 is at a reduced pressure and the pressure ofthe main chamber 9, which is under compression, is applied to the valve52 on the side of its return spring 54 penetrating in the bore 50 viathe orifice 75, the valve 52 is held on the base of the bore 50 wherethe transportation channel 62 emerges and the communication passage 48itself also remains closed. Therefore, the compressed high-viscosity oilpasses from the working chamber 9 to the working chamber 8 via therestricted permanent communication passage 76. Since the cross sectionof this passage 76 has a relatively small diameter, the damping producedis then a high-gradient damping, which corresponds to the start of thecurve shown by the graph in FIG. 8.

When the relative displacement of the bodies 1 and 2 increases further,from a predetermined value of the pressure of the oil in the chamber 41,the oil of said chamber 41 in the transportation channel 61 willdisplace the viscous restriction valve 51 relative to the base on whichsaid valve 51 bears and against the return spring 53. This displacementis all the greater when the relative displacement of the two rigidbodies 1 and 2 is itself large. As the valve 51 is lifted, the groove 59is gradually positioned opposite the communication passage 47, thus moreor less opening the said passage 47, according to whether it is or isnot exactly opposite the two ends of said passage 47 on the walls of thebore 49. For the same reason as above, the valve 52 remains bearing onthe base of the bore 50 where the transportation channel 62 emerges andcloses the restricted communication passage 48. The oil will thus passessentially from the working chamber 9 to the chamber 8 via thecommunication passage 47. During the phase of gradual increase in thepassage cross section by the groove 59, the damping produced is lessthan before, which corresponds to the second part of the curve shown bythe graph in FIG. 8, that is to say to the part which descends from thevertex Fal, corresponding to a stress peak obtained for a relativedisplacement d1 resulting in a control pressure Pcl in the secondarychamber 41.

If the relative displacement of the two bodies 1 and 2 increasesfurther, beyond the position in which the groove 59 is exactly at thelevel of the communication passage 47, the viscous restriction valve 51will gradually close said communication passage 47 again and give riseto a high-gradient damping, which corresponds to the third part of theabovementioned curve (rising part of the curve on the righthand side ofthe graph).

During the displacement of the viscous restriction valve 51 towards theend of the bore 49 towards which the capsule 55 which carries the spring53 is mounted, said viscous restriction valve 51 pushes the oil back viathe conduit 74 towards the chamber 8 which is under reduced pressure,which fills up. Since the rod 3 has a diameter which is identical oneither side of the damping piston 4, in the chambers 8 and 9, the volumeby which said compressed chamber 9 is reduced is equal to the volume bywhich the chamber 8 under reduced pressure increases. Therefore, thevolume by which the chamber 9 is reduced corresponds substantially tothe volume of oil which passes from said chamber 9 to the chamber 8 viathe communication passages 47 and 76. The small volume of oil pushedback by the viscous restriction valve 51 via the conduit 75 thusconstitutes a surplus in said chamber 8. It is discharged by means ofthe transfer channel 64 and the transportation channel 62 in thesecondary chamber 42, which is also at reduced pressure. Thedifferential valve 68 is, in fact, in the open position away from itsseat 66, since the valve 67 to which it is connected via the connectionrod 69 is held so as to bear on its seat 65 by the differential pressureexerted on it by the compressed oil of the chamber 9, which pressure istransmitted thereto in the chamber 70 by means of the transfer channel63.

During such a relative displacement in extension of the two rigid bodies1 and 2 relative to one another, the displacement of the end of the mainrod 3 in the auxiliary chamber 6 releases a potential volume for theoil. This variation in volume is compensated for by a transfer of oilfrom compensation chamber 10 in said auxiliary chamber 6 by means of theelements of the compensation circuit which form the various channels 35and 43 to 46.

Moreover, FIG. 1 shows that, for reasons of symmetry, the respectiveroles of the working chambers 8 and 9 and of the various elementsassociated therewith are interchanged with respect to one another if thetwo rigid bodies 1 and 2 undergo a relative displacement with respect toone another, no longer in extension but in compression. The only majordifference is that a portion of the oil of the auxiliary chamber 6 isthen pushed back via the free end of the main rod 3, through thechannels 43 to 46 and 35, into the compensation chamber 10. The bellowswalls 11 of said chamber 10 expand, the reference rod 99 emergingslightly more from the plate 81.

Referring again to FIG. 8, it can be seen that the principle offunctioning, described above, of the damper can be summarised in thefollowing manner: A pressure Pc in the control damper corresponds to arelative displacement D of the two rigid bodies 1 and 2 relative to oneanother; a balance position of one of the viscous restriction valves ofthe damper corresponds to this pressure Pc; an opening cross section ofthe restricted communication passage with which said valve is associatedcorresponds to this balance position, said opening cross section beingdetermined in advance by construction; a damping stress Fa on the mainrod 3 and the damping piston 4 corresponds to this opening crosssection. The damping law achieved is essentially determined by theprofile of the viscous restriction valves. Such a damper structure thusmakes it possible, by constructional play, to achieve virtually anydamping law as a function of the damped dynamic displacement.

In the example which has just been described, the damping law is anoptimised law more particularly adapted for elastic-return struts withintegral damping for the blades of a rotor of an aerodyne with a rotarywing unit. Three damping zones may be distinguished, as a function ofthe dynamic displacement:

a first zone corresponds to small displacements D to d1 (from 0 to moreor less 2 mm), for which it is desirable to have a high damping gradientin order to counteract the ground resonance phenomena;

in a second zone or intermediate zone, which corresponds to the averagerelative displacements between d1 and d2 (between 2 and 4 mmapproximately in absolute value), the level of damping is low, which is,in particular, desirable for stabilised flight configurations;

in a third zone, which corresponds to major displacements beyond d2(greater than 4 mm approximately in absolute value), that is to say inthe case of a load factor, a high damping gradient is again necessary.

FIGS. 5 and 6 show an elastic-return strut with integral damperaccording to a second embodiment of the invention. For the elements ofthis second embodiment which are to be found in the first embodimentdescribed above, the same reference numerals have been used, increasedby 100.

These figures show that a strut according to this second embodiment ofthe invention comprises two rigid bodies 101 and 102. The rigid body 102is integrally attached to a main rod 103 which substantially passesthrough the other rigid body 101 lengthwise. A damping piston 104 ismounted on this rod 103, sliding in translation in a main cylinder 105provided inside the rigid body 101. The inner diameter of the maincylinder 105 is slightly greater than the outer diameter of the piston104 in order to delimit a permanent peripheral viscous restriction play.The axes of the main rod 103, the damping piston 104 and the maincylinder 105 are substantially coincident. The damping piston 104delimits, with two end partitions 114 and 200, two working chambers 108and 109 in the main cylinder 105. These two working chambers 108 and 109are themselves inside a compensation chamber 110 and an auxiliarychamber 106, the partition 114 partially delimiting the compensationchamber 110, the partition 200 partially delimiting the auxiliarychamber 106.

The main rod 103 has a cylindrical recess 134 passing through itlengthwise, extended by a channel 135, A secondary cylinder 139 isdelimited in the cylindrical recess 134 between two disks 136 and 201.The main rigid damping body 101 is also integrally attached to asecondary rod 132, part of which extends axially into the main rod 103.This secondary rod 132, in said main rod 103, passes through theauxiliary chamber 106, the working chambers 108 and 109, and partiallythe compensation chamber 110. This secondary rod 132 also passes throughthe secondary cylinder 139 lengthwise, its free end being disposed inthe channel 135. A secondary piston 140, which delimits, in thesecondary cylinder 139, two secondary chambers 141 and 142, is mountedon this secondary rod 132. Said secondary piston 140 has an outerdiameter which is slightly smaller than the inner diameter of thesecondary cylinder 139 in order to delimit, as in the preceding example,a peripheral viscous restriction play which determines the controlpressure law generated by the control damper formed by the secondarypiston 140 and the secondary cylinder 139. The secondary rod 132 alsohas passing through it axially, from the central portion of the piston140 to its end in the channel 135, a secondary channel 143 which emergesinto said channel 135. The piston 140 has a supplementary channel 144extending from said secondary channel 143 to the periphery of saidpiston 140 passing radially through it. This channel 144 allows, withthe viscous restriction play which exists between the secondary piston140 and the secondary cylinder 139, communication between the secondarychambers 141 and 142 and the channel 135.

The rod 103 has a supplementary channel 145 passing radially through it,placing said channel 135 and the compensation chamber 110 incommunication. The compensation chamber 110 and the auxiliary chamber106 are connected together by channels 202 and 203 which pass throughthe rigid body 101 in its wall delimiting the main cylinder 105. Theauxiliary chamber 106, the compensation chamber 110, the channels 202and 203, the channel 135 and the supplementary channels 143 to 145 areelements of a compensation device intended to compensate for the thermalexpansions of the hydraulic fluid as well as the variations in thevolumes containing said fluid. This compensation device also comprises acharging chamber 204 disposed, in the extension of the compensationchamber 110, of the working chambers 108 and 109 and the auxiliarychamber 106, on the side of said chamber 106 opposite the workingchamber 109. The base 133 of the auxiliary chamber 106 has a restrictedpassage 205 passing through it which connects said auxiliary chamber 106and the charging chamber 204.

The main rod 103 passes right through the auxiliary chamber 106. The endof said rod 103 opposite to its end via which it is integrally attachedto the rigid body 102 is integrally attached to the base 133 of saidchamber 106. The cylindrical recess 134 emerges outside the main rod 103in the charging chamber 204. The secondary rod 132 is mounted on asheath 206 integrally attached to a plate 207 of the rigid body 101.Said sheath 206 and the secondary rod 132 are integrally attached to oneanother by the interaction of their complementary screw threads as wellas by the interaction of the screw thread of the secondary rod 132 withthe screw thread of a counternut 208 which is locked on the free end ofthe sheath 206. The plate 207 carries a fixing lug 186 on which a pivot187 is mounted. The sheath 206 is also associated, in its centralportion, with a ring 209 which can slide relative to said sheath 206,said ring being associated with an annular dynamic seal 210 ensuring theleaktightness between said ring 209 and said sheath. A frustoconicalelastic wall 211, widening from said ring 209 as far as a cylindricalbody 212, is mounted on this ring 210. The base 133, the cylindricalbody 212, the recess 134, the disk 136 and the flexible wall 211together delimit the charging chamber 204.

The damping piston 104 also comprises, in its thickness, two bores 149and 150 (see FIG. 6) in which the valves 151 and 152, respectively,slide, pushed back by springs 153 and 154, respectively, towards thebase of said bores 149 and 150. The axes of these two bores 149 and 150are disposed in a cluster in a cross section of the damping piston 104.Towards the ends of the bores 149 and 150 where said bores 149 and 150are closest to one another, the springs 153 and 154 are supported bycapsules 155 or 156 disposed in housings 157 or 158 extending said bores149 or 150. Transportation channels 161 and 162 connecting said bores149 or 150 to the chambers 141 and 142 emerge, respectively, at theother ends of said bores 149 or 150. Transfer channels 163 and 164connecting, respectively, said transportation channels to the chambers109 and 108 emerge in these transportation channels 161 and 162. Thesetransfer channels 163 and 164 are, respectively, associated withdifferential valves 167 and 168 connected together by a connecting rod169 and controlling the opening and closing of said transfer channels163 and 164. Towards their ends which carry the capsules 155 and 156,the bores 149 and 150 emerge, by means of conduits 174 and 175,respectively, into the working chamber 108 and the working chamber 109.

The main rod 103 is integrally attached to the rigid body 102 by meansof an axial boss 213 which carries a fixing lug 183 associated with apivot 185. The rod 103 is itself integrally attached to a tubularsection 214. This tubular section 214 is connected to a tubular section215 which laterally extends the partition 114 and substantiallysurrounds said tubular section 214 via a sleeve 216 in an elasticmaterial which adheres in a leaktight manner to the outer and innersurfaces, respectively, of the sections 214 and 215. At the opposite endof the strut, the cylindrical body 212 is, in the same manner, connectedto a tubular section 217, which surrounds it, via a second elasticsleeve 218 which also adheres in a leaktight manner to the sections 212and 217. The two elastic sleeves 216 and 218 each delimit with,respectively, the partitions 114 and 200, the bases 133 and 219, and aportion of the tubular sections 215 and 217, the compensation chamber110 and the auxiliary chamber 106.

The part formed by the section 215 and the partition 114 is integrallyattached to the central portion of the main body 101, which portiondelimits the main cylinder 105 by means of flanges 220 and 221 withwhich said tubular section 215 and said main cylinder 105 are equipped,respectively. These two flanges 220 and 221 are held relative to oneanother by means of a screw 222 whose head bears on the flange 220 andwhose screw thread interacts with the screw thread of bores passingthrough the flange 221. Similarly, the part formed by the tubularsection 217 and the partition 200, the main portion of the body 101 andthe plate 207 are integrally attached to one another by means of flanges223, 224 and 225, with which they are respectively equipped, saidflanges being associated with screws 226 which pass through bores withwhich said flanges 223 to 225 are equipped, the screw thread of thesescrews 226 interacting with the screw thread of the bores of the flange223, the head of said screws 226 bearing on the outer surface of theflange 225, the body of said screws 226 extending through the bores withwhich the flanges 224 and 225 are equipped.

Moreover, the main cylinder 105 is associated with a purging screw 194intended to permit the filling of the working chambers 108 and 109, forexample with a highly viscous oil.

The functioning of such a strut is substantially identical to thefunctioning of the strut described above, in terms of its dampingportion, and to the functioning of the struts described in theApplicant's Patent Application FR-2,629,163, to which reference willadvantageously be made for any further details on this subject. When thetwo rigid bodies 101 and 102 are moved away from their balance position,they are returned towards one another via the elastic sleeves 216 and218, all the movement being damped by the damper incorporated in thestrut, the damping law being substantially a function of the relativedisplacement of the two bodies 101 and 102 relative to one another.

In the two examples described hereinabove, it is observed that thecontrol of the viscous restriction openings of the damper by a devicelinked to the dynamic displacement of this damper, and comprising asecondary or control damper delivering a hydraulic control pressurecontrolling said viscous restriction openings, makes it possible toobtain, by virtue of this decoupling, a viscous restriction law of thedamper which is totally independent of the resulting damping force.

In these two examples, also, in order to compensate for the variationsin volume due to thermal expansion or accidental leakages of thehydraulic oil, the compensation chamber is in communication, on the onehand, directly with the control damper via a conduit made in the centerof the piston, in order to filter the flow pulses at the use frequency,and, on the other hand, by means of the control damper and by means ofdifferential valves, with the main damper.

It will be clearly understood that, although the damper according to theinvention has been described here within the framework of anelastic-return strut with integral damping, in particular for a blade ofan aerodyne with a rotary wing unit, it may have numerous otherapplications.

What is claimed is:
 1. An elastic-return strut with integral damping, ofelastohydraulic type, in particular for controlling the alternatedangular movements of the blades of a rotor of an aerodyne with rotarywing unit in the plane of rotation of said blades, comprising;two rigidmembers (1, 2; 101, 102) each equipped with articulation means (84, 85,86, 87; 184, 185, 186, 187) intended to connect one of the rigid members(1; 101) to a first part, such as a blade or a member for joining saidblade to the boss of the rotor, and the other rigid member (2; 102) to asecond part, such as said boss of the rotor; at least one elastic-returnmember (77; 216, 218) comprising a sleeve in an elastically deformablematerial adhering in a leak tight manner via its inner and outer lateralsurfaces, respectively, between inner and outer rigid tubular sections(13, 78; 214, 215, 217, 212) respectively, substantially coaxial withrespect to the axis of the strut and each integrally attached to one,respectively, of the two rigid members (1, 2; 101, 102), so that thesleeve (77; 216, 218) is deformed in shear, when the two rigid members(1, 2; 101, 102) are displaced relative to one another substantiallyaccording to the general axis of the strut and when the sleeve (77; 216,218) exerts an elastic-return action on said rigid member (1, 2; 101,102) which tends to bring them back into an initial relative position;and a hydraulic damper comprising, in a main body (1; 101), two workingchambers (8, 9; 108, 109) with a volume which is variable in oppositedirections, said working chambers (8, 9; 108, 109) containing a mainviscous fluid intended to pass from one to the other via at least onerestricted communication passage (47, 48, 76; 147, 148, 176), throughthe action of the thrust of a damping piston (4, 104) being displaced insaid body (1; 101), an opening cross section of at least onecommunication passage (47, 48; 147, 148) being controlled by thepositioning, relative to said passage (47, 48; 147, 148), of a viscousrestriction valve (51, 52; 151, 152) on which a control fluid acts,wherein said control fluid is contained in at least one secondarychamber (41, 42; 141, 142) delimited in a secondary cylinder (39; 139)by a control piston (40, 140) mounted slidably in said secondarycylinder (39, 139), said secondary chamber (41, 42; 141, 142) beingassociated with a transportation channel (61, 62; 161, 162) of thecontrol fluid from said secondary chamber (41, 42; 141, 142) as far asthe viscous restriction valve (51, 52; 151, 152), through the action ofthe control piston (40; 140), said secondary cylinder (39, 139) and saidcontrol piston (40; 140) being integrally attached, one to the main body(1; 101), the other to the damping piston (4; 104) such that a dampinglaw of said control fluid positioning said viscous restriction valve islinked to the relative dynamic displacements of said main body and saiddamping piston, the main body (1; 101) of said damper forming one of thetwo rigid members, the damping piston (4;104) of said damper beingintegrally attached to the other (2; 102) of said rigid members, saiddamping being dependent upon said relative dynamic displacements of saidmain body and said damping piston.
 2. The strut as claimed in claim 1,wherein a secondary movable rod (32 ; 132) and a main movable rod (3;103) are mounted, one on the articulation means connecting one of saidrigid members to the first part, the other on the articulation meansconnecting the other of said rigid members to the second part.
 3. Thestrut as claimed in claim 34, wherein the sleeve (77) adheres, via itsinner surface, to the outer surface of a tubular wall (13) of the mainbody (1; 101) of the damper.
 4. The strut as claimed in claim 34, whichcomprises two elastic sleeves (207, 218) adhering via their innersurface to the outer tubular surface of sections (212, 214) integrallyattached to a main movable rod (103), and, via their outer surface, tothe inner tubular surfaces of sections (215, 217) integrally attached tothe main body (101) of the damper.
 5. The strut as claimed in claim 4,wherein the elastic sleeves (216, 218) respectively at least partiallydelimit a compensation chamber (110) and a auxiliary chamber (106). 6.The strut as claimed in claim 3, wherein a secondary movable rod (132)is connected to a main movable rod (103) via at least one flexibleelastic-return wall (211).
 7. The strut as claimed in claim 6, whereinthe flexible elastic-return wall (211) has a frustoconical form wideningfrom a sheath (206), in which the secondary movable rod (132) ismounted, as far as the inner wall of a tubular section (212).